US10712360B2 - Differential charge transfer based accelerometer - Google Patents
Differential charge transfer based accelerometer Download PDFInfo
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- US10712360B2 US10712360B2 US16/142,349 US201816142349A US10712360B2 US 10712360 B2 US10712360 B2 US 10712360B2 US 201816142349 A US201816142349 A US 201816142349A US 10712360 B2 US10712360 B2 US 10712360B2
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- 238000005259 measurement Methods 0.000 claims abstract description 110
- 239000003990 capacitor Substances 0.000 claims abstract description 65
- 230000003071 parasitic effect Effects 0.000 claims abstract description 25
- 230000001133 acceleration Effects 0.000 claims description 42
- 238000000034 method Methods 0.000 claims description 24
- 238000004513 sizing Methods 0.000 claims description 3
- 238000012545 processing Methods 0.000 description 10
- 230000007812 deficiency Effects 0.000 description 5
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
- G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
- G01P15/125—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by capacitive pick-up
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/18—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration in two or more dimensions
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
- G01R27/26—Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
- G01R27/2605—Measuring capacitance
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P21/00—Testing or calibrating of apparatus or devices covered by the preceding groups
Definitions
- Integrated accelerometers which utilize capacitive structures are well known in the art, and are becoming more prevalent in consumer electronics such as smart phones, tablets and so forth.
- Such accelerometers typically comprise Micro Electromechanical Systems (MEMS) technology, wherein comb-like structures have sets of conductive fingers which are spaced apart and interleaved, with one set of fingers typically movable relative to the other.
- MEMS Micro Electromechanical Systems
- the conductive fingers of said structures move closer or further from each other, with an associated change in measured voltages.
- signal processing circuitry typically within the integrated accelerometer, calculates the acceleration force applied.
- a comb-like structure is utilized for each orthogonal direction in a three dimensional coordinate system.
- the capacitive comb is just one possible MEMS implementation used by the prior art, with many alternatives known for using MEMS to obtain capacitive readout due the displacement of a proof mass.
- a z-axis accelerometer may be realized by using the parallel plate capacitance between two plates which move relative to one another.
- FIG. 1 illustrates circuitry for some prior art accelerometers in a block diagram schematic.
- Block 1 . 1 represent the comb-like structures, with C 1 the capacitance between a set of fingers 1 . 2 and a movable set 1 . 3 , and C 2 the capacitance between another set of fingers 1 . 4 and the movable set of fingers 1 . 3 .
- C 1 and C 2 changes accordingly, with C 1 which typically increases and C 2 which decreases for movement along one axis, and vice versa.
- the series string of C 1 and C 2 is typically connected between a pulsed positive supply rail 1 . 5 and a correspondingly pulsed negative rail 1 . 6 , with the voltage at the centre of the series string fed via an interconnection 1 .
- the voltage at 1 . 10 represents the delta or change in capacitances C 1 and C 2 . If no movement occurs, and C 1 and C 2 do not change from their calibrated values, the voltage on 1 . 10 stays at zero. Any change in C 1 and C 2 should result in a voltage above or below zero. For instance, if C 2 decreases, and C 1 increases accordingly, more voltage will fall over C 2 , resulting in a voltage above zero at 1 . 10 . Conversely, for an increase in C 2 , the voltage at 1 . 10 should change to below zero.
- Amplifier block 1 . 7 feeds an amplified value of the voltage at 1 . 10 to an analog-to-digital (A/D) block 1 . 8 via interconnect 1 . 11 , with block 1 . 8 also supplied from positive and negative rails.
- a digital value of the voltage at 1 . 11 is communicated by the A/D via a serial or parallel connection 1 . 12 to a digital signal processor (DSP) 1 . 9 , which provides an indication of calculated acceleration, or values proportional to it, at 1 . 13 .
- DSP 1 . 9 may be supplied with a single positive rail and ground.
- One of the drawbacks of some prior art accelerometers is the requirement for both a positive and negative supply rail, as this requires additional circuitry and semiconductor real estate.
- Another drawback is the complexity of the circuitry, with an amplification stage, a digital conversion stage and a signal processing stage, wherein these stages typically conserve a fair amount of power.
- the prior art does not make direct use of the underlying mechanism present in said comb-like structures. When said conductive fingers move relative to one another, the capacitance of the structures and the relative amounts of stored charge changes. It may be advantageous to directly measure the differential change in stored charge and capacitance with a charge transfer technique which have low relative power consumption.
- fF femto Farad
- aF 10 ⁇ 18 F
- prior art capacitive structure MEMS accelerometers may be improved by effective compensation for parasitic capacitances of the MEMS structure, as well as differential capacitance offset compensation with a large range, easing production and integrated circuit matching requirements.
- the present invention teaches the use of charge-transfer (CT) circuitry and methods to directly measure the differential change in stored charge and capacitance of conductive structures due to acceleration experienced by said structures. More specifically, the present invention teaches that a CT circuit may be used to measure the difference between charge stored in a first and a second capacitance of a conductive structure or structures due to acceleration experienced by said structure or structures, wherein said first and second capacitances may be arranged in a back-to-back combination, with the two distal ends of said combination which may be alternately and in complimentary manner connected to a positive supply and to ground respectively.
- CT charge-transfer
- a second distal terminal of said second capacitance may be connected to a positive supply rail, and vice versa.
- the alternate and complimentary connection of the distal ends or terminals of said back-to-back combination of capacitances may be performed repetitively, and may form part of a CT process or method used by said CT circuit.
- the present invention teaches that measurement of said differential change in charge stored in the capacitances, and thereby of the capacitances themselves, may be realized as follows.
- the distal, first terminal of said first capacitance may be connected to ground via a first controllable switch, and the distal, second terminal of said second capacitance may be connected to a positive supply rail via a second controllable switch.
- the second terminal of the first capacitance may be tied to the first terminal of the second capacitance to form a common terminal.
- a connection may be made via another controllable switch from said common terminal to an input of a current mirror circuit.
- the current mirror circuit may be used to scale charge being transferred to a reference capacitor in a charge transfer measurement circuit, as disclosed in U.S. Pat. No. 8,659,306 to Bruwer et al, wherein said reference capacitor may be used to accumulate transferred charge up to a certain threshold level, or for a certain period, as is known in the art.
- said first terminal of the first capacitance may be connected to said positive supply rail via a third controllable switch and the second terminal of the second capacitance may be connected to ground via a fourth controllable switch, with said common terminal which may be connected via said another controllable switch to the input of the current mirror circuit.
- Said first and second phases of the first measurement may be alternately repeated until the voltage on said reference capacitor reaches a predetermined threshold, or for a predetermined period, with the number of charge transfers, or counts, which may be indicative of said differential change in capacitance and the amount of charge stored.
- a sample-and-hold circuit may be used to maintain the voltage at the input to said current mirror circuit at a constant level during repetitions of said first, or charge, phase of the first measurement, preferably, but not necessarily, at a value equal to the value at the input of said current mirror at the end of the second, or transfer, phase of the first measurement. This may be used to ensure that no netto charge, or a derivative thereof, due to parasitic capacitances present between said common terminal and ground is transferred to said reference capacitor.
- the first terminal of said first capacitance may be connected to the positive supply rail via the third controllable switch, and the second terminal of the second capacitance may be connected to ground via the fourth controllable switch, with said common terminal which may be connected to the input of said current mirror circuit via said another controllable switch.
- the first terminal of the first capacitance may be connected to ground via the first controllable switch
- the second terminal of the second capacitance may be connected to the positive supply via said second controllable switch
- the common terminal may be connected to the input of the current mirror circuit via said another controllable switch.
- Said first and second phases of the second measurement may be alternately repeated until the voltage on said reference capacitor reaches a predetermined threshold, or for a predetermined period, with the number of charge transfers, or counts, which may be indicative of said differential change in capacitance.
- a sample-and-hold circuit may once again be used to maintain the voltage at the input to said current mirror circuit at a constant level during repetitions of said first, or charge, phase of the second measurement, preferably, but not necessarily, at the value present at the input of said current mirror at the end of the second, or transfer, phase of the second measurement. This may be used to ensure that no netto charge, or a derivative thereof, due to parasitic capacitances present between said common terminal and ground is transferred to said reference capacitor.
- said first and second capacitances may be viewed as two mutual capacitances effectively connected in parallel and to the input of said current mirror and charge transfer measurement circuit.
- the amount of charge stored in each capacitance should therefore be directly dependent on its size.
- the described and disclosed alternate connection of the terminals of said capacitances to the supply voltage and to ground may be seen as being equivalent to driving the transmitter terminals of said two mutual capacitances out of phase, with the result that the charge from one capacitance is subtracted from the charge of the other capacitance. Consequently, only the difference between the charge stored in the first capacitance and that stored in the second capacitance, or a charge delta, may be available for transfer to said charge transfer measurement circuit via the current mirror input.
- charge delta If the charge delta is positive, charge may enter the input diode of the charge transfer measurement circuit, and a charge proportional to it may be added to the reference capacitor of said circuit. If the charge delta is negative, current flow may be blocked by the input diode of the charge transfer measurement circuit, and no charge may be added to said reference capacitor.
- performing a first measurement as disclosed may be seen as being equivalent to subtracting the charge stored in said second capacitance from that stored in said first capacitance.
- performing a second measurement as disclosed may be seen as being equivalent to subtracting the charge stored in said first capacitance from that stored in said second capacitance.
- first and second measurements of the present invention as described above may be used as follows to determine both the amount of differential change between said first and second capacitances and which capacitance increased. For example, consider the case where said capacitances change so that the first capacitance is larger than the second. During said first measurement, subtracting the charge stored in the smaller, second capacitance from that stored in the larger, first capacitance should therefore result in a small surplus of charge, which may be transferred to said reference capacitor.
- the number of charge transfers needed to fill said reference capacitor up to a threshold during the first measurement may therefore be used as indication of the amount by which the first capacitance increased, or the acceleration experienced.
- subtracting the charge stored in the larger, first capacitance from that stored in the second, smaller capacitance should result in a small deficiency of charge. Due to said diode like behaviour of the current mirror input, charge may not be transferred out of the current mirror circuit to compensate for the charge deficiency. Consequently, the amount of charge stored in said reference capacitor will typically stay constant, and the second measurement may time out before the reference capacitor reaches it threshold voltage.
- both the first and second measurements should result in very little or no change in the amount of charge stored in the reference capacitor, and a time-out situation for the charge transfer measurement process.
- the measurement system may indicate that no acceleration was experienced, since both the first and second measurement delivered a no change, or time-out result.
- the preceding disclosure may be used to measure acceleration experienced by a measurement system, as an exemplary application, using charge transfer measurements, a current mirror circuit and a reference capacitor.
- a reference capacitor may not be required, and may be replaced with circuitry that measures current flow out of said current mirror circuit, and integrates the current over time.
- the present invention teaches that it may be necessary to perform both said first measurement and said second measurement before declaring a result, to accommodate the case where no or negligible acceleration was experienced.
- the present invention teaches that parasitic capacitance cancellation (PCC) apparatus and methods as disclosed in U.S. Pat. No. 8,395,395 and in U.S. Pat. No. 8,659,306 by Bruwer et al may be used by the present invention to compensate for unwanted capacitances and changes in capacitance.
- the present invention further teaches specifically that it may be possible to compensate for parasitic capacitances between the above first terminal of the first capacitance and ground, and between the above second terminal of the second capacitance and ground by utilizing sufficiently large transistors to drive said terminals.
- a sample-and-hold circuit may be used to ensure that the voltage on the common node (shared by said second terminal of the first capacitance and said first terminal of the second capacitance) stays constant during the charge phase of the charge transfer cycle, for example at the voltage value of the common node at the end of a transfer phase of a previous charge transfer cycle.
- a sample-and-hold op-amp circuit may be used in conjunction with a plurality of switches to charge a sample capacitor to a voltage present on said common node during one time period, and to drive said common node to the same voltage, using the voltage on the sample capacitor as input to the op-amp, during another period.
- differential capacitive accelerometer structures typically exhibit a difference of around 100 fF between said first and second capacitances due to manufacturing tolerances, while the change in capacitance due to acceleration which has to be measured may be on the order of a few aF. (These values should not be construed as a limit to the present invention, as they are merely provided to aid clarity of disclosure.)
- the difference between said first and second capacitances typically cause a large offset in the signal being processed to determine acceleration.
- such an offset may be removed or cancelled out by using a microprocessor algorithm to control a highly accurate, digitally variable capacitor, wherein said variable capacitor may be used to extract or change the amount of charge from a reference capacitor during certain periods of a charge transfer measurement cycle. Further, the variable capacitor may also be used with a current mirror structure or structures to facilitate said charge extraction.
- a distinct structure which comprises differential first and second capacitors may be utilized for each axis or direction, as an example.
- a single structure which comprises multiple differential capacitor pairs may be utilized to measure acceleration, or another parameter, along a plurality of axes or directions, according to the present invention.
- the present invention also teaches that methods and means as disclosed may be used to measure acceleration, or another parameter, along a plurality of axes or directions, for example along X, Y and Z axes, in a sequential or concurrent manner. That is, acceleration, or another parameter, may be measured, for example, along an X-axis first, then along a Y-axis and then along a Z-axis, or the acceleration may be measured simultaneously along all three axes. In the case of a sequential measurement, the present invention is not limited in terms of the pattern or sequence of measurement.
- FIG. 1 shows a typical prior art measurement system used to monitor acceleration with a serially connected capacitive structure.
- FIG. 2 shows an exemplary embodiment of the present invention for monitoring acceleration with effectively parallel capacitive structures and using a switching structure, charge transfer measurements and current mirror circuitry.
- FIG. 3 shows qualitative transmitter electrode signals for a first and second measurement made with an exemplary embodiment of the present invention.
- FIG. 4 shows a table summarizing typical results for measurements made with exemplary embodiments of the present invention.
- FIG. 5 shows an exemplary embodiment of the present invention with compensation for common junction parasitic capacitance as well as offset compensation.
- FIG. 2 presents an exemplary embodiment of the present invention.
- Block 2 . 1 represents a comb-like structure, as is known in the art of MEMS based accelerometers, wherein two primary capacitances C 1 and C 2 are realized, connected in a back-to-back configuration.
- each of these capacitances may have parasitic, or unintended, capacitances, represented by C p11 , C p12 , C p21 and C p22 respectively.
- capacitances C 1 and C 2 may be measured as mutual or projected capacitances, with C 1 having a transmitter electrode Tx 1 and receiver electrode Rx 1 , and C 2 having a transmitter electrode Tx 2 and receiver electrode Rx 2 , respectively.
- Said parasitic capacitances are typically formed between a transmitter or receiver electrode and ground, as shown.
- Rx 1 and Rx 2 are connected together and represent the common electrode of said comb-like structure used for measuring acceleration.
- the transmitter electrode Tx 1 of C 1 may be connected via a controllable switch S 1 to a supply voltage 2 . 2 , or it may be connected via a controllable switch S 2 to ground 2 . 3 , as shown.
- the transmitter electrode Tx 2 of C 2 may be connected to said supply voltage via a controllable switch S 3 , or it may be connected to ground via a controllable switch S 4 .
- a controllable switch S 5 may be used to connect the common point formed by Rx 1 and Rx 2 to the input 2 . 5 of a current mirror 2 . 4 .
- S 5 is continually closed, although it should not be limited to this state.
- current flowing at output terminal 2 . 6 may be dependent on the current at input terminal 2 . 5 and a predetermined mirror ratio.
- Current flowing into input 2 . 5 may result from charge being transferred from either C 1 or C 2 , wherein said charge transfer may occur due to the controlled switching of switches S 1 to S 4 , as will be elaborated on.
- output 2 . 6 of current mirror 2 . 4 is connected to input 2 . 8 of current mirror 2 . 7 , which may result in a scaled current flowing via output 2 . 9 of mirror 2 . 7 .
- Said scaled current which may be larger or smaller than the current that flows into input 2 . 5 of mirror 2 . 4 , may be applied to a reference capacitor Cr 1 via a controllable switch S 6 , wherein the number of charge transfers to fill Cr 1 to a predetermined level, or which occurs within a predetermined period, may be monitored and used as measurement metric, for example as indication of the amount capacitance, as is known in the art.
- the present invention is further embodied in the qualitative transmitter electrode waveforms of FIG. 3 , which presents an exemplary manner to control the circuitry of FIG. 2 for a differential, CT based measurement of the change in C 1 and C 2 , wherein said change may be due to acceleration experienced by the measurement system.
- a first measurement may be made with transmitter waveforms as illustrated at 3 . 1 .
- Axis 3 . 2 represents amplitude, with level 3 . 5 indicating when the supply voltage is connected to a particular transmitter electrode, and level 3 . 6 indicating when the particular transmitter electrode is connected to ground.
- Axes 3 . 3 and 3 . 4 represent time.
- transmitter electrode tx 1 may be connected to ground via switch S 2 ( FIG. 2 ), and transmitter electrode tx 2 may be connected to the supply voltage via switch S 3 ( FIG. 2 ).
- transmitter electrode tx 1 may be connected to the supply voltage via switch S 1 ( FIG. 2 ), and transmitter electrode tx 2 may be connected to ground via switch S 4 ( FIG. 2 ).
- said first charge phase ⁇ 11 and said second transfer phase ⁇ 12 may subsequently be alternately repeated during the remainder of said first measurement until the reference capacitor Cr 1 ( FIG. 2 ) reaches a predetermined threshold, or for a predetermined period.
- phase repetitions in FIG. 3 is only provided as an example, and should not be seen as limiting.
- said first measurement as described and depicted in FIG. 3 may be seen as being equivalent to subtracting the charge stored in C 2 ( FIG. 2 ) from that stored in C 1 ( FIG. 2 ), and if the difference is positive, it or a derivative thereof may be transferred to said reference capacitor. If said difference is negative, no charge may be transferred to said reference capacitor due to blocking by the input diode of the current mirror circuit 2 . 4 ( FIG. 2 ).
- a second measurement may need to be made before declaring a result, as depicted in exemplary manner in FIG. 3 at 3 . 7 .
- transmitter electrode tx 1 may be connected to the supply voltage via switch S 1 ( FIG. 2 ), and transmitter electrode tx 2 may be connected to ground via switch S 4 ( FIG. 2 ).
- This may be followed by a second transfer_phase ⁇ 22 of the second measurement during which transmitter electrode tx 1 may be connected to ground via switch S 2 ( FIG. 2 ) and transmitter electrode tx 2 may be connected to the supply voltage via switch S 3 ( FIG. 2 ).
- the first charge phase ⁇ 21 and the second transfer phase ⁇ 22 of said second measurement may subsequently be alternately repeated until the reference capacitor Cr 1 ( FIG. 2 ) reaches a predetermined threshold, or for a predetermined period.
- the number of phase repetitions illustrated is only provided as an example, and should not be seen as limiting.
- said second measurement as described and depicted in FIG. 3 may be seen as being equivalent to subtracting the charge stored in C 1 ( FIG. 2 ) from that stored in C 2 ( FIG. 2 ), and if the difference is positive, it or a derivative thereof may be transferred to said reference capacitor. If said difference is negative, no charge may be transferred to said reference capacitor due to blocking by the input diode of the current mirror circuit 2 . 4 ( FIG. 2 ).
- one repetition of phase ⁇ 11 and phase ⁇ 12 may form a charge transfer cycle.
- one repetition of phase ⁇ 21 and phase ⁇ 22 may form a charge transfer cycle.
- ⁇ Q is the delta of charge transferred during a particular charge transfer.
- FIG. 4 an exemplary table is presented which summarises the above, with three cases or scenarios considered.
- a first capacitance C 1 of an embodiment of the present invention is larger than a second capacitance C 2 , for example due to acceleration experienced by said embodiment.
- a first measurement and a second measurement may need to be made with apparatus and methods as described earlier.
- a delta of charge ⁇ Q 1 may be transferred during each charge transfer cycle or phase, wherein said delta may be proportional to the amount by which C 1 is larger than C 2 during Case 1 .
- the charge transfer counts Ct 1 at the end of said first measurement should be proportional to the amount by which C 1 is larger than C 2 .
- the counts Cr 1 for the second measurement of Case 1 should be equal to Ctmax, as a timeout would typically occur.
- associated processing circuitry may determine that C 1 is larger than C 2 , seeing that only the first measurement resulted in charge transfer, and said processing circuitry may also use the number of charge transfers Cr 1 performed during the first measurement to determine the amount by which C 1 is larger than C 2 . For example, in an acceleration measurement application, this may be indicative of the amount of acceleration experienced. It should further be noted that for Case 1 , the result achieved after the first measurement may be sufficient for some applications, without a requirement for a second measurement as described.
- a capacitance C 2 in an embodiment of the invention is larger than a capacitance C 1 , for example due to acceleration experienced by the embodiment.
- a first measurement which may be deemed equivalent to subtracting charge stored in C 2 from that stored in C 1 , no charge should be transferred, seeing that C 2 is larger than C 1 and said subtraction should result in a deficiency of charge which may typically not be transferred out of the current mirror circuit, as described earlier. Therefore, the counts Ct 2 for the first measurement of Case 2 should be equal to Ctmax, as a timeout would typically occur.
- a delta of charge ⁇ Q 2 may be transferred during each charge transfer cycle or phase, wherein said delta is proportional to the amount by which C 2 is larger than C 1 during Case 2 . Therefore, the charge transfer counts Ct 2 at the end of said second measurement should be proportional to the amount by which C 1 is smaller than C 2 .
- associated processing circuitry may determine that C 2 is larger than C 1 , seeing that only the second measurement resulted in charge transfer, and said processing circuitry may also use the number of charge transfers Ct 2 performed during the second measurement to determine the amount by which C 2 is larger than C 1 , similar to before.
- capacitances C 1 and C 2 are equal. For example, in an acceleration measurement application, this may occur when no acceleration is experienced, and the comb-like structure mentioned earlier maintains the spacing between the various sets of electrodes or conductive fingers. As shown in FIG. 4 , no charge should be transferred during either the first or second measurement. An associated processing circuit may deduce from the fact that the charge transfer counts Ct 3 equalled Ctmax for both the first and second measurement of Case 3 that C 1 is equal to C 2 .
- FIG. 5 presents another exemplary embodiment of the present invention at 5 . 1 .
- the circuit depicted in FIG. 5 is similar to the circuit of FIG. 2 , with like reference designators referring to like members.
- a square wave generator 5 . 2 is connected between a supply rail +V and ground 5 . 3 , and outputs a square wave at node 5 . 5 .
- Generator 5 . 2 is not limited, and may utilize any relevant circuitry, including that depicted in FIG. 2 .
- Capacitances C 1 and C 2 may represent a capacitive comb structure, as discussed earlier, with terminals Tx 1 and Tx 2 respectively the driven terminals of C 1 and C 2 , and terminals Rx 1 and Rx 2 forming a common terminal 5 . 6 .
- An inverting element 5 . 4 may be used to ensure that Tx 2 is driven in anti-phase to Tx 1 , e.g. when Tx 1 is driven at supply rail +V, Tx 2 is connected to ground potential and vice versa.
- a number of parasitic capacitances exist in practical comb, and other, capacitive structures, represented by C p11 , C p21 , C p12 and C p22 .
- the present invention teaches that sufficiently large transistors (e.g. in square wave generator 5 . 2 and in inverter 5 .
- Parasitic capacitances C p11 and C p21 should not impede the measurement process if said transistors allow sufficient current flow to adequately charge C 1 and C p11 or C 2 and C p21 .
- Switches S 7 and S 8 may be used to charge sample capacitor 5 . 11 during a transfer phase such that the voltage at node 5 . 12 is equal to the voltage at 5 . 6 , preferably to the voltage at 5 . 6 at the end of the transfer phase.
- switches S 7 and S 9 are used during the charge phase to drive the voltage at node 5 . 6 with op-amp 5 . 10 to equal the voltage at node 5 . 12 .
- switch S 7 is continually closed, although this need not necessarily be so.
- use of the sample-and-hold circuit constituted by op-amp 5 . 10 , capacitor 5 . 11 and controlled switches S 8 and S 9 should ensure that practically no netto charge stored in parasitic capacitances C p12 and C p22 , or a derivative thereof, is transferred to the reference capacitor, thereby cancelling the effect of said parasitic capacitances.
- the measurement process for the circuit depicted in FIG. 5 is similar to that described above for FIG. 2 , with two charge transfer measurements made, wherein the one measurement is characterised by terminal Tx 1 first being driven high, then Tx 2 , followed by a number of charge transfer cycle repetitions, and wherein the other measurement is characterised by terminal Tx 2 first being driven high, then Tx 1 , followed by a number of charge transfer cycle repetitions.
- FIG. 5 also includes a third current mirror structure 5 . 9 , which may be used with switches S 10 and S 11 along with digitally controlled variable capacitor 5 . 14 to compensate for any offset due to a difference between C 1 and C 2 , for example when no acceleration is experienced by the circuit.
- a compensation algorithm which may execute on a microprocessor, microcontroller, computer or another computing platform, may be used to adjust or select the value of variable capacitor 5 . 14 .
- Switch S 11 may be used to selectively charge variable capacitor 5 . 14 to the supply rail voltage +V, or to another voltage.
- Switch S 10 may be used to selectively allow a current due to the discharge of variable capacitor 5 . 14 , or another current, to flow into the input of current mirror 5 . 9 .
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Abstract
Description
Qr=Cr*Vt; and Ct=Qr/ΔQ
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